268 HEINZ SCHUSTER 



TABLE VII 

 Optical Activity op TMV-RNA° 



Material 



TMV-RNA 

 TMV-RNA 

 TMV-RNA 

 Degraded TMV-RNA 



a Taken from A. Gierer, Z. Nalurforach. 13b, 477 (1958). 



the hyperchromic effect. Only after a longer incubation with enzyme, 

 corresponding to that resulting in an increase in U V absorption, is there a 

 decrease in optical rotation. These two effects are shown in Fig. 5 as a 

 function of the time of incubation with enzyme. From a comparison of 

 both curves, one may observe an obvious parallelism in the two effects. 



Birefringence. In DNA, according to the Watson-Crick model, the purine 

 and pyrimidine rings are at right angles to the length of the molecule. Since 

 there is a negative birefringence for DNA, 78 and a positive birefringence 

 for TMV-RNA as well as for intact TMV, 79 it is likely that the bases in 

 RNA are parallel to the long axis of the molecule. The magnitude of the 

 birefringence suggests a highly ordered structure. 



d. Constitution and Biological Activity 



The above sections have discussed the evidence that isolated RNA from 

 TMV is infectious, and that only an RNA molecule having a molecular 

 weight of approximately 2 X 10 6 is capable of initiating TMV infection, 

 whereas smaller fragments are biologically inactive. We now turn to the 

 problem of macromolecular RNA as a structure capable of transmitting 

 genetic function. It is possible to investigate the problem of alterations in 

 genetic functions induced by controlled alterations in structure and com- 

 position and to see how one may alter the molecule without loss of function. 

 It may be possible to distinguish between alterations which lead to no 

 changes in biological activity, those resulting in a complete loss of function, 

 and those resulting in an altered function, leading to mutant forms. 



(1) Action of Physical Agents. The action of X-rays. RNA isolated from 

 TMV which has been X-irradiated prior to the isolation of the nucleic acid 

 has a viscosity which is lower than that of RNA isolated from unirradiated 

 control TMV. 80 This suggests that the lethal action of X-rays is due to a 



78 M. H. F. Wilkins, R. G. Gosling, and W. E. Seeds, Nature 167, 759 (1951). 



79 R. E. Franklin, Biochim. et Biophys. Acta 18, 313 (1955). 



80 M. A. Lauffer, D. Trkula, and A. Buzzell, Nature 177, 890 (1956). 



